RPM v Pressure on Aerial

I know this question has probably been asked before but can you guys tell me the difference in rpm v pressure. We have a 75' E One aerial. It has a digital throttle and all. Of course you select which mode you would like and throttle up. On a regular house fire where the aerial device will not be used you would use what? And on a large fire with both hoeslines and aerial in use you would use what? Im used to our engine where you just charge the lines and go. Why will the pump run away? Sorry for all the questions. This wasnt covered in our App Operator Class and I havent taken Aerial yet.

Just to be clear, you took an apparatus/pump operator class and they didn't teach you the difference between the two modes? Did the apparatus used in the class not have an electronic pressure governor?

No the book we used didnt have it in there. Not that I recall I will look again when on shift. The pumper we used had a manual relief valve. The aerial has a choice between pressure and rpm. Im not trying to sound like an idiot. Pressure is for handlines and rpm when using the waterway?

What you're describing is a pressure governor, which is basically a cruise control for the pump. Think of it this way - the PG can look at only two gauges on the pump panel, and it can only look at one of these gauges at a time. Those two gauges are the tachometer (RPM) and master pressure gauge (PSI).

When it's in the RPM mode, it will maintain the RPM you currently have it set on, no matter what the increased or decreased demand on the pump is. For example: you're pumping a single handline at 150psi, and you introduce 40psi into the pump when you open the hydrant. Since you've told the PG not to vary the RPM at all, where is that 40psi going? Into that hoseline! Now you've got a 190psi pump discharge pressure and some ticked off guys on the end of the hose.

Now, let's use that same example, but you've set the PG into the PSI mode. Now the PG is going to maintain the pressure that the master discharge gauge is reading, and it does this by varying the RPM of the motor automatically. So in this case, you've got one handline and you've got 150psi on it. However, when you open that 40psi hydrant in this case, the pump will recognize it doesn't have to work as hard to maintain 150psi PDP, and will automatically throttle the engine down since it doesn't have to work as hard to maintain that 150psi. Conversely, if you shut off that 40psi hydrant, it will throttle the engine back to up maintain 150psi.

What you're describing is a pressure governor, which is basically a cruise control for the pump. Think of it this way - the PG can look at only two gauges on the pump panel, and it can only look at one of these gauges at a time. Those two gauges are the tachometer (RPM) and master pressure gauge (PSI).

When it's in the RPM mode, it will maintain the RPM you currently have it set on, no matter what the increased or decreased demand on the pump is. For example: you're pumping a single handline at 150psi, and you introduce 40psi into the pump when you open the hydrant. Since you've told the PG not to vary the RPM at all, where is that 40psi going? Into that hoseline! Now you've got a 190psi pump discharge pressure and some ticked off guys on the end of the hose.

Now, let's use that same example, but you've set the PG into the PSI mode. Now the PG is going to maintain the pressure that the master discharge gauge is reading, and it does this by varying the RPM of the motor automatically. So in this case, you've got one handline and you've got 150psi on it. However, when you open that 40psi hydrant in this case, the pump will recognize it doesn't have to work as hard to maintain 150psi PDP, and will automatically throttle the engine down since it doesn't have to work as hard to maintain that 150psi. Conversely, if you shut off that 40psi hydrant, it will throttle the engine back to up maintain 150psi.

HTH...

Lt,
I would add to use RPM when in relay, unless you are the attack engine, as a line of rigs in relay with PGs in PSI mode will cause havoc.

Lt,
I would add to use RPM when in relay, unless you are the attack engine, as a line of rigs in relay with PGs in PSI mode will cause havoc.

I disagree.

If the governor is set in RPM, it will add X amount of pressure to Y amount of water.

If the flow suddenly halts, then the friction loss in the system disappears. It’s that friction loss that required us to use a relay in the first place. Without that friction loss, you turn a relay operation into a long-distance multi-stage pump.

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TXGP: I have the good fortune to instruct pump I & II as well as Rural Fireground Water Movement for Pa. State Fire Academy local level training. Over the years (since 1974) long relay operations have changed since we now have relay relief valves and most engines are being controlled with electronic pressure governors. Several times with RFGWM classes we have been able to establish long relays of a mile or more. In every case, operators are instructed to NEVER shut down rapidly. Relay pump operators get hand cuffed to pump panels, and eyes glued to both master gauges. The problem with today’s LDH and the modern pressure governors is the reaction time to changes in pressure and volume. If we were pumping to iron pipe, then your theory about Press. X Volume would be correct and all pumps could be placed into the control of the pressure governor. The reality of modern supply hose is the fact that the hose expands with pressure, and every change in volume passing through the system takes time to work its way from the supply engine to final discharge. Here is what happens when a long relay gets every engine operating on the pressure governor.
Say we get a hose line to shut down on the fireground (100 gal out of say 1500 GPM) when running a 6 engine relay of 5” hose with 900 ft. between engines. The discharge tries to climb on the attack engine (#6), and the pressure governor cuts back causing the intake to rise, so the governor cuts back even more. The 900 ft. of hose has slightly expanded delaying the pressure increase to the #5 relay engine. It finally gets the signal and throttles down, but the intake pressure has risen (#’s 1,2,3, & 4 haven’t reacted since the change hasn’t made it back there yet). This wave of pressure increases takes time to work its way back from the attack engine all the way back to the supply engine. Each engine in turn except the supply has over compensated for the change in intake pressure and they are now all at too low a pressure (too low RPM) and needs to throttle back up to meet the demand of 1400 GPM. The friction loss drops to 117 PSI, and correct discharge pressure needs to be 137 psi at each relay engine, but this pressure wave is now passing back and forth from engine to engine causing pressure governors to fluctuate.
By having the relay engines operated in RPM mode, the drivers react slower to the changes and are less likely to over adjust the rpm before the engine nearer the source sees the increasing discharge pressure. While not a fool proof method, it does provide a better control of the relay pressures. The best way to control a long relay with many engines is to put a drop tank at the attack engine and allow the relay to be open ended. It is possible to “Hot Tap” the attack engine by pulling the intake pressure to zero on the compound and remove the off-side suction cap and attach the hard sleeve on the fly. Thus the decrease in water usage will simply flow into the drop tank and the rest of the relay engines will never see any difference in throttle requirements as lines are started and shut down on the fireground.

SMOKEEATERFF: With a pressure governor, the priming operation needs to be completed and all air removed from the intake and pump BEFORE switching to pressure. This includes operating from the tank. When I exit the cab after getting the pump in gear, I set the chock block and then open the tank to pump. Set the governor to RPM and increase to around 900 to 1,000 RPM. (depends upon the transfer case gear ratio) Pull the primer handle or hit the push button. As soon as you start to see pressure on the discharge gauge, start opening the tank fill and stay on the primer valve for a few more seconds. ( 5 to 10) The high flow going back to the tank will help to clear the trapped air. Then cut the tank fill back to provide a little circulation through the pump. You do not need the pressure governor control until a second line is in operation. By the time the second line is advanced, all the trapped air should be cleared from the pump. Then switch to the preset pressure or the SOP pressure for the preconnect line. The next time you will need to stay glued to the panel is when the hydrant or relay line gets charged. I try to switch to rpm for the change-over and then go back to pressure mode. Try to bleed the trapped air through the intake valve slowly so it gets sent out to the nozzle as little bubbles instead of breaking the prime in your pump. Once switched over to the supply line, slowly close the tank to pump, but refill your tank as soon as you can. When operating the aerial, it is not absolutely necessary to set the control to pressure, but at high flows, if the ladder operator shuts off the nozzle you will experience high pressures if you are in RPM mode. These high pressures might be more than the waterway is designed to reach.
Pump runaway occurs when the pump looses prime (air lock) or is starved (cavitation) for water. When cavitation or loss of water occurs, the system is supposed to throttle up for a period of time trying to maintain the pressure and then go to idle and flash a warning. The real danger for the nozzle team is when the pump auto throttles up looking for water, and then gets hit with a sudden supply. At this point the pressure can jump well above 300 psi depending upon how the supply is regained and where the rpm is when it happens. DO NOT switch from pressure to RPM when the governor has throttled up. Instead, press idle. War stories can sometimes illustrate the consequences of making a wrong move. Within the past year, we had a pump operator who was unfamiliar with one of our pressure governor engines. At a working structure, the pump was placed into “Preset” before all the air was removed from the pump. When the attack line opened up before entering the building. (Our standard SOP) The slug of air was drawn into the pump, breaking the prime and causing the engine to throttle up. The operator reacted to the increasing throttle by pushing the RPM button after the engine had reached about 1800 rpm. His thought was to throttle down, but he never got the chance. This action locked the motor at 1800 and when prime was regained (a couple seconds after he hit the button) the pressure climbed to over 600 psi. Blew the preconnect hose line right beside the operator striking him across the bridge of his nose and knocking him to the ground. In the 15 or so seconds it took the Lt. to shut down the line and press idle, the pressure surge took out the check valves for the foam system, blew apart the control valves for the relay relief valve and generally destroyed a preconnect valve that was forced shut aganst the flow.
Pump castings are tested to 600 psi, but not the control equipment that gets attached later. Best advice I can give is to play with your equipment. The more you operate valves and hydraulics the less problems you will have when there is a need for it.

I know this question has probably been asked before but can you guys tell me the difference in rpm v pressure. We have a 75' E One aerial. It has a digital throttle and all. Of course you select which mode you would like and throttle up. On a regular house fire where the aerial device will not be used you would use what? And on a large fire with both hoeslines and aerial in use you would use what? Im used to our engine where you just charge the lines and go. Why will the pump run away? Sorry for all the questions. This wasnt covered in our App Operator Class and I havent taken Aerial yet.

In Pressure (PSI) mode the engine will run away when your engine is unable to maintain the Pump Discharge Pressure with the water supply provided.

The simplest example is if someone shuts down the hydrant after you get lines in service. When everything is running you're getting 40 PSI from the hydrant and the PG is maintaining 150 PSI PDP. If the hydrant shuts down you loose that 40 PSI and the PG ups the throttle to maintain 150 using tank water. If you run low of tank water the PG is going to keep increasing the throttle. It doesn't know there is no water.. it just knows PDP < Preset.

By having the relay engines operated in RPM mode, the drivers react slower to the changes and are less likely to over adjust the rpm before the engine nearer the source sees the increasing discharge pressure. While not a fool proof method, it does provide a better control of the relay pressures. The best way to control a long relay with many engines is to put a drop tank at the attack engine and allow the relay to be open ended.

I haven't thought this through completely, but couldn't the same affect be achieved by having the attack engine leave "Tank Fill" open? If the attack engine is in PSI mode and the hose line shuts down the extra pressure gets sent to the tank. Eventually the tank will fill and drain out the top.

Have the relay pumpers in RPM mode manually maintaining a set residual and let the attack engine deal with changes on the front end.

If the hydrant shuts down you loose that 40 PSI and the PG ups the throttle to maintain 150 using tank water.

I know this has been dabated before on here, but since we close our tank-to-pump when a water supply has been established, it would take conscious decision of the pump operator to open the TTP to use tank water in this scenario.

Thanks for the help guys. Like you said the best thing to do is get out there and flow some water. Our Lt. knows his stuff but feels like he doesn't need to share with us. If you don't know it oh well.

I know this has been dabated before on here, but since we close our tank-to-pump when a water supply has been established, it would take conscious decision of the pump operator to open the TTP to use tank water in this scenario.

True.. and in that case the PG would "run away" when the hydrant is closed, not when the tank's dry. There are pros/cons to keeping T2P closed but doesn't change the example much. (Unless I'm missing something?)

The only major difference is when the pressure governor would react to the loss of incoming pressure. If the pump operator wasn't married to the pump panel, the engine would throttle up for 5 seconds looking for water, and then return to idle (assuming the TTP was closed). If the TTP was open, obviously the pump would get some water until the tank was depleted, and then it would do it's throttle up/idle routine.

Not really a big deal at all, it's something that seems to come up every time pump theory is discussed.

I haven't thought this through completely, but couldn't the same affect be achieved by having the attack engine leave "Tank Fill" open? If the attack engine is in PSI mode and the hose line shuts down the extra pressure gets sent to the tank. Eventually the tank will fill and drain out the top.

Have the relay pumpers in RPM mode manually maintaining a set residual and let the attack engine deal with changes on the front end.

Leaving the tank fill open would not be enough to handle the potetially large volume of water that would need to be expelled. Tank fills frequently are anything from 1" to 2" which would not be capable of handling really large quantities of water. But with the larger tank fills, it would add enough flow to require the governor to ramp the engine up considerably and it would cause dumping of all that excess water out onto the ground.

I know this has been dabated before on here, but since we close our tank-to-pump when a water supply has been established, it would take conscious decision of the pump operator to open the TTP to use tank water in this scenario.

All of our newer trucks have operational (I.E. working) flapper valves. This is why we leave our TTP valve open with an incoming water source.